Implement a parser for dates

Implement a parser for dates

In order to parse user specified strings containing date and time info
a thread-safe parser is implemented. This is needed for example to
interpret configuration parameters (e.g. gc.pruneexpire where need to
parse strings like "2 weeks ago"). The parser is thread-safe by caching
SimpleDateFormat instances in a ThreadLocal cache.

Native git has a parser called approxidate which is able to interpret a
huge number of formats ("1 year ago", "tea time", ...). Ideally JGit
should be able to parse the same strings as native git but for now this
parser understands the following subset:

"now"
"yesterday"
"(x) years|months|weeks|days|hours|minutes|seconds ago"
"yyyy-MM-dd HH:mm:ss Z" (ISO)
"EEE, dd MMM yyyy HH:mm:ss Z" (RFC)
"yyyy-MM-dd"
"yyyy.MM.dd"
"MM/dd/yyyy"
"dd.MM.yyyy"
"EEE MMM dd HH:mm:ss yyyy Z" (DEFAULT)
"EEE MMM dd HH:mm:ss yyyy" (LOCAL)

Change-Id: Iccb66dadb60da13104e73140e53d5e2de068369c