Performance improvements

[NiklasRosenstein]

On my Windows machine, time nppm has an average real time of little less than a second. The recently added caching in FilesystemResolver helps only marginally (around 5%). Here I collect a list of things that might be the cause to the slow start time of the nppm command (which is largely caused by the Node.py implementation).

• Reloading pkg_resources when the Context is entered introduces around 0.100s and 0.200s
• Many requests to the filesystem are made when resolving files (eg. upiter_directory() and FilesystemResolver.resolve())
• Importing pip introduces around 0.350s to 0.500s (!) (we can't really avoid that though)