Maven dependency management improvements

* add maven-enforcer-plugin, enable "dependency convergence" rule: http://maven.apache.org/enforcer/enforcer-rules/dependencyConvergence.html (ensure that there are no inherited artifacts with different versions)
* add missing dependencies to <dependencyManagement> section
Maven plugin configuration improvements

* move common plugin configuration to <pluginManagement> section
* set version for all plugins
* replace deprecated `pom.version` with `project.version`
* replace deprecated `pom.url` with `project.url`
Imports in proto files should work for multi-module project

Maven plugin can search for imported proto files in the project's runtime classpath elements
Use standard UTF-8 encoding, and allow for backwards compatable decoding.
Merge pull request #38 from kshchepanovskyi/feature/proto-import-from-dependency

Imports in proto files should work for multi-module project
Update StringSerializer constants
Merge branch 'master' into fix/dependency-management

Conflicts:
	pom.xml
	protostuff-maven-plugin/pom.xml
	protostuff-me/pom.xml
Fix build errors after merge with master (missing version for plexus in maven configuration)
Merge pull request #42 from kshchepanovskyi/fix/dependency-management

Update maven configuration
Fixed maven configuration: standalone compiler jar build

https://github.com/protostuff/protostuff/issues/45
Fixed maven configuration: standalone compiler jar build

Upgrade jarjar plugin, use dependency from repository instead of local lib
Merge pull request #46 from kshchepanovskyi/refactoring/standalone-compiler

Fix maven configuration: standalone compiler jar build
Fix broken package name (after mass rename to io.protostuff)
Switch order of String deserializers
Added comment with regards to replacement characteR
Merge pull request #49 from kshchepanovskyi/master

Fix broken package name (after mass rename to io.protostuff)
Revert constant changes, add property to prevent String scanning on decode
Require maven version 3.2.3

All previous maven versions are not secure - they use http connection to maven central instead of https
Require maven version 3.2.3

Upgrade maven dependencies for protostuff-maven-plugin
Merge pull request #39 from kikinteractive/java8-string-encoding

Java 8 Encoding/Decoding Fix