Move documentation of user-defined counters

It was mistakenly moved to an internal module as part of a refactoring.
Small documentation tweaks
Shorten complicated sentence in documentation
Remove conditional export of GC counter

Previously par_tot_bytes_copied was only exported if base-6.4 or later
was used to compile ekg. Now it's always exported and uses
parAvgBytesCopied (which has the same meaning as parTotBytesCopied) when
ekg is compiled against older versions of base.
Removing warning on base < 4.6
Remove mention of version number from the UI

It was too hard to keep in sync and didn't really serve any purpose.
Add release notes for 0.3.1.2 after the fact
Add a note on security
Correctly bind the server to the right host name

This allows you to e.g. only listen for connections to localhost. This
fixes #11.
Change basic example to not blow the stack

It was using sum from the Prelude which is implemented using a
non-strict left fold, causing it to blow the stack.
Whitespace only
Add release notes for 0.3.1.3
Remove another mention of version in UI
Bump version number to 0.3.1.3
Factor out and polish counter, gauge, and label serving.

Handlers are no longer triggered by a prefix of their path.
Generalize partitionGcStats
Generalize readAllRefs
Add a way to get a snapshot of all metrics
Move parseHttpAccept to Snap module
Add a travis-ci config file