Fix warnings in the build output

E.g.:

Package natbib Warning: Citation `buterin2013ethereum' on page 1 undefined on i
nput line 99.
Package natbib Warning: Citation `dagger' on page 14 undefined on input line 12
07.
LaTeX Warning: Reference `ch:Terminology' on page 2 undefined on input line 114
.
Underfull \hbox (badness 10000) in paragraph at lines 204--205
[]\OT1/cmr/m/n/9 Having in-tro-duced the ba-sic con-cepts be-hind
LaTeX Warning: Reference `app:rlp' on page 3 undefined on input line 208.
Overfull \vbox (4.76pt too high) has occurred while \output is active [4]
Overfull \hbox (15.41824pt too wide) detected at line 980
[][]\OML/cmm/m/it/9 W\OT1/cmr/m/n/9 (\OML/cmm/m/it/9 w; []\OT1/cmr/m/n/9 ) \OMS
/cmsy/m/n/9 ^^Q []
Package natbib Warning: There were undefined citations.
[37] (./Paper.aux)
Package rerunfilecheck Warning: File `Paper.out' has changed.
(rerunfilecheck)                Rerun to get outlines right
(rerunfilecheck)                or use package `bookmark'.

LaTeX Warning: There were undefined references.
LaTeX Warning: Label(s) may have changed. Rerun to get cross-references right.
This is BibTeX, Version 0.99d (TeX Live 2013/Debian)
The top-level auxiliary file: Paper.aux
The style file: plainnat.bst
Database file #1: Biblio.bib
Warning--empty journal in back2002hashcash
Warning--empty journal in buterin2013ethereum
Warning--empty journal in dagger
Warning--empty journal in hashimoto
Warning--empty journal in FowlerNollVo1991FNVHash
Warning--empty journal in lerner2014randmemohash
Warning--empty journal in colouredcoins2012rosenfeld
Warning--empty journal in mastercoin2013willett

To do:

[x] #520: \tiny and \small in math mode:
[x] #543: Warning--empty journal
[x] Package natbib Warning: Citation `...` on page # undefined on i nput line #. LaTeX Warning: Reference `...' on page # undefined on i nput line #. These are normal. See here.
[x] Underfull/overfull \hbox or \vbox. Read this. All overfull hbox warnings have been fixed and thus removed with #548.
[ ] LaTeX Warning: Reference `ch:Terminology' on page 2 undefined on input line 114. These could be replaced with \linkdest{Terminology}.

ethereum / yellowpaper

Fix warnings in the build output #541