Rename {{contains}} helper to {{includes}}

[jelhan]

This renamed {{contains}} helper to {{includes}} as discussed in #365. A deprecation warning is added for the old name.

Additional I have refactored to code:

• Remove code only needed to support Ember versions, which fall back to EmberArray.contains() if EmberArray.includes() is not available. EmberArray.contains() has been removed in 2.x. EmberArray.includes() is available in all supported version.
• Avoid converting the array passed in multiple times to EmberArray if an array of needles is provided.
• Use Array.every() (or EmberArray.every()) instead of Array.reduce() to improve readability and performance.
• Use same code path for single needle and array of needles to reduce complexity. This may have a bad performance impact as it requires creating an interim array if a single needle is passed. But the performance impact should be small enough to be acceptable.

Closes #365

[snewcomer]

Thank you for the PR!!

[Alonski]

@jelhan @snewcomer Not to be picky but the README wasn't updated with this change 😜

[bertdeblock]

Created #397 for the docs part.