Support for node v6.0

Merge pull request #6 from 2gis/update-nan

Update nan to v2.4.0 and fix code to build on node v6

0.6.4

Merged: #5 (fix return after librsvg errors)

0.6.3

• Added testing for resulting pngs (thanks to @physikerwelt)

Fix build for BSD/Solaris

0.6.2

bump version

0.6.1

Updated travis.yml to build succesfully with gcc-4.8+

Node v4 support

v0.6.0 uses updated version of NAN to provide compatibility with latest nodejs v4 release. Also node v0.10 and v0.12 are supported.