Remove profile declarations

No longer needed.
Add support for namespace-based metacommands

And more general namespace support
Dynamic 'do' provider for namespaces

Using a namespace such as 'rebar3 lfe new' will look for the 'new'
command in 'lfe' namespace without the need for a 'do' provider to be
registered in that namespace.

Manually checked that 'rebar3 as <profile> <namespace> command' works,
as well with 'default' and 'do'.

Test suite still needed.
Fix up programmatic interface to rebar3

Now supports arbitrary commands
Tests and fixes for namespaces
Merge pull request #70 from rebar/provider-namespaces

Provider namespaces