Make coverage reports es6

[magicmark]

• Switched to nyc from istanbul
• Moved coverage report levels (need to investigate why they dropped in some cases)
• Changed some test imports to import from src rather than lib

(Used to look like this, now they look like this

test:quick and tests-only are kinda useless now but unsure if safe to deprecate (major bump?)

[ljharb]

nyc has more accurate coverage than istanbul-cli, so it's ok if they dropped a little bit.

[magicmark]

@ljharb rebased + made changes. I guess babel-node isn't needed since nyc is using babel-register to transpile tests.

[magicmark]

@ljharb rebased again

passing with node 6 and 7:

v6.3.1:
=============================== Coverage summary ===============================
Statements   : 91.06% ( 163/179 )
Branches     : 70.31% ( 45/64 )
Functions    : 57.89% ( 44/76 )
Lines        : 90.8% ( 158/174 )
================================================================================

v7.7.3:
=============================== Coverage summary ===============================
Statements   : 91.06% ( 163/179 )
Branches     : 70.31% ( 45/64 )
Functions    : 56.58% ( 43/76 )
Lines        : 90.8% ( 158/174 )
================================================================================