Make PicoTorrentCore cross-platform

[vktr]

This is probably the biggest PR since we separated PicoTorrentCore and PicoTorrent. The basic idea behind this refactor is to make PicoTorrentCore cross-platform.

Here is the checklist,

• Remove filesystem abstraction.
• Move configuration and environment to the client.
• Make the session class take a session_configuration instance to support configuring the session.
• Generate the session_configuration from the client configuration.
• Remove std::wstring dependency from PicoTorrentCore.
• Add a Platform Adaption Layer (PAL) which abstracts the specific platform functions needed for PicoTorrentCore.
• Add a PAL for Windows.
• Move logging implementation to the client.
• Add a Travis CI build which builds PicoTorrentCore only.
• Reload session_configuration when the client saves its preferences.

Closes #149