v0.5.0: First usable release

This release constitutes the first usable release. Some limitations
apply, the greatest one probably being that Linux + GCC is the only
supported combo thus far (see #51).

This release is not considered stable; bookwyrm has now entered the
first big bug-squashing stage. All issues listed in #53 and otherwise
tagged with v0.6.0 shall be resolved before next minor release.

Highlights of this release are:
    * dropped some dependencies;
    * replaced termbox with ncurses;
    * add plugin for Library Genesis;
    * a lot of wrestling with CPython to bypass segfaults;
    * implemented downloading of items via libcurl;
    * TONS of refactoring and general improvements.