Fixed incorrect tag name behaviour.

When a tag name contains a dot character, Rails interprets this as the
separator for a formatted route and splits the tag name into the section
before the dot which is interpreted as the expected :tag parameter, and
the section after the dot which is interpreted as an unexpected :format
parameter.

This commit restores correct behaviour in the event that a tag name
contains a dot character and includes spec coverage.

Fixes #93.
Addressed broken Atom format.

Tags can now be viewed in HTML and Atom format regardless of whether the
tag name contains a dot character or not. The regular expression that
matches tag names currently allows upper/lower case letters, numbers,
dots, dashes, underscores and spaces (URL encoded or not).

Updated relevant specs. Moved routing specs into posts_routing_spec.rb
as this seems like a more fitting place for them.
Updated for spacing consistency.
Filter tag names to contain only expected chars.

Tag names can now only contain alphanumeric, underscore, space, dot and
dash characters. Other characters will be filtered out, ampersands will
be expanded to 'and'. This limitation has been enforced to avoid
complications with routing where tag names are involved.

Updated specs accordingly.
Fixed post preview feature.

The posts preview feature was broken by the advent of Rails 4 strong
params. This commit restores the feature back to working order.

Updated specs.
Merge pull request #95 from gaelian/post-preview-fix

Fixed post preview feature.
Merge pull request #94 from gaelian/dot-in-tag-name-fix

Fixed incorrect tag name behaviour.