Added link to IETF-hosted RFC 2119 spec.

Went with the hosted version of RFC 2119 that Wikipedia links to. It's not terribly pretty, but it works.
Fix typo
Merge pull request #59 from oguzbilgic/patch-1

Fix typo
Make version format consistent

Elsewhere in this section we use `0.y.z`. In this place, we use `0.x.x`
This commit fixes that.

Closes #18
Change sentence to remove inapplicable "Remember"

Closes #31
Merge pull request #21 from slippyd/master

Linking to RFC 2119
Change "be comprised of" to "comprise"
Merge pull request #73 from vergenzt/master

Change "be comprised of" to "comprise"
Updated to RC2, with new definition for build metadata.
Removed an extraneous slash
Add description of build metadata

Build metadata is a completely optional way to tack on metadata to a
version  that does not affect the public API or precedence while still
be compliant with SemVer. It's not unlike a comment.

This allows folks to build on additional meaning or behavior that's
meaningful to their custom systems without making any claims about the
public version. For example, folks might use it for cache busting,
layering custom behavior on top of SemVer, bookkeeping to tie when and
where a build was made, etc.
Omit build metadata from precedence rules

Looks like a big change, but all I did was remove Build metadata from
the precedence rules.
Removed the plus sign and added examples

The `+` seemed out of character with the rest of the spec. Following the
example of the pre-release section, we just spell out "sign" and provide
examples for those who don't know what a plus sign is. :)
Fixed grammar mistake

Changed "be comprised of only" with the correct "comprise only".
Bonus, it's less verbose.
Add space between "hypen" and regex.
Merge pull request #77 from mojombo/build-metadata-identifier

Build metadata identifier
Merge pull request #1 from mojombo/master

Update to latest 。