Elixir exercise bank account

This exercise involves accessing shared data from multiple threads. In Elixir
this requires understanding communication by messages and how to write a state
storage process.
Merge pull request #860 from pminten/bank-account-for-elixir

Elixir exercise bank account
Add ruby 'hamming' exercise

This is a re-do of the point-mutations exercise, which is deprecated.
There's currently no good method for deprecating exercises, so this
has been temporarily moved to the end of the curriculum list.
Rename bank acount example
Change context of text to user's page
Merge pull request #863 from Tonkpils/fix-context

Change context of text to user's page
rust rna-transcription - rust 0.8 no longer has map_chars, use replace instead
rust - check exercises automatically with travis (rust 0.8)
rust - word-count exercise