Persist choice from 'nvmw use [...]'

[yoannmoinet]

With these changes nvmw use [iojs|node] will persist in system path accross cmd's sessions. As quickly discussed in #42.

Here are the steps :

• clean path from special characters
• remove previous .nvmw paths
• add back special characters
• determine which iojs or node to get
• set PATH and NODE_PATH in System Path (won't work if System Path is more than 1024 characters, silly windows)
• set PATH and NODE_PATH in current cmd's session

Let me now if this is useful to anyone else.