Parse all attribute level properties

[jrideout]

For our test set at least, all TNEFObject level attributes are now being parsed correctly. If these attributes are not otherwise used, they are placed in the msgprops attribute of the TNEF object.

I also have some cleanup mixed in (apologies):

• Checked in the test file from @TZanke umlaut.tnef
• Migrated nearly all use of bytes_to_int to use the struct based methods
• I removed the double encode/decode from strings attributes

[codecov[bot]]

Codecov Report

Merging #38 into master will increase coverage by 0.04%. The diff coverage is 90.36%.

@@            Coverage Diff            @@
##           master     #38      +/-   ##
=========================================
+ Coverage   90.16%   90.2%   +0.04%     
=========================================
  Files           5       5              
  Lines         915     960      +45     
  Branches       84      93       +9     
=========================================
+ Hits          825     866      +41     
- Misses         67      69       +2     
- Partials       23      25       +2

Impacted Files	Coverage Δ
tnefparse/util.py	75.75% <100%> (+2.32%)	:arrow_up:
tnefparse/mapi.py	94.07% <85%> (+0.32%)	:arrow_up:
tnefparse/tnef.py	89.33% <91.52%> (-1.33%)	:arrow_down:

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[jrideout]

@petri other than finishing up the few items in #26, this concludes all the short-term objectives I had. I have some ideas on refactoring, but nothing urgent.