Fix traceback print on None for |location|

This happens when the user invokes a GoTo* command on a file with no compilation
flags.
Update install.sh

Added more efficient option to detect number of cores on running system via nproc
Merge pull request #240 from izx64/patch-1

Update install.sh
Refactoring the way install.sh checks for commands
Removing some dead code
Refactoring out some vim.eval calls

s:completion_start_column should not be evaled in Python code. That was a
stopgap measure that ended up lasting far longer than intended.
Fix typo introduced in refactor
Fix traceback on <C-Space> semantic invoke

This was caused by yesterday's refactoring. I forgot to update one call site.

Fixes #242
Possible fix for random hang on Vim close

Reparse would take the clang lock and then possibly call Destroy while still
holding the lock. Destroy would try to take that same lock, and the mutex is not
recursive. Unpleasantness ensues.

I _think_ this is the root cause of #218, but I can't be sure. Such is life with
threads.

Fixes #218.
Updating vimdoc
Make caching optional in the omnicompleter