Add maven-enforcer plugin

Adds configuration specification for the maven-enforcer plugin and a
default binding to the validate lifecycle phase. This will cause the
maven-enforcer:enforce target to be run at the start of each maven
build, using whichever rules are active.

As configured, these rules are always on:
* enforceBytecodeVersion
* banCircularDependencies

These rules are on by default but can be disabled by setting the system
property: scijava.enforce.skip:
* requireReleaseDebs
* banDuplicateClasses
* requireMavenVersion
* requirePrerequisites
* requireReleaseDeps

As some of these properties may use maven and jvm versions, these have
been converted to their own properties.
Merge pull request #12 from scijava/maven-enforcer

Enables the maven-enforcer plugin by default. Rules are separated into two categories: "loose" that are always on, and "strict" that are on when scijava.enforce.skip is not set. This allows us to disable certain rules, for example when we are building development versions of code.