Add a postinstall script for node4 compat

[nono]

Bcrypt@0.8.1 is compatible with node 0.x.y Bcrypt@0.8.5 is compatible with node 0.10.40+

We want cozy-proxy to works on:

• node 0.10.25 (the version installed by default on ubuntu)
• node 0.10.29 (the version installed by default on debian)
• the current stable release of node 0.10, 0.12, 4 and 5

So, we have to use the postinstall feature of npm to check the version of node and install a version of bcrypt compatible with it.

[frankrousseau]

Thank you @nono