Add Sentinel annotation and AspectJ support

[sczyh30]

Describe what this PR does / why we need it

• Add Sentinel annotation definition (in sentinel-core)
• Add AspectJ support for Sentinel annotation

Does this pull request fix one issue?

Resolves #11

Describe how you did it

The @SentinelResource contains:

• resource name (required)
• resource entry type (optional)
• fallback method (optional)
• block handler method (optional)

We've implemented an aspect with the pointcut around any methods annotated with @SentinelResource, and wrap with Sentinel API.

[codecov-io]

Codecov Report

Merging #43 into master will decrease coverage by 0.12%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master      #43      +/-   ##
============================================
- Coverage     46.74%   46.61%   -0.13%     
  Complexity      548      548              
============================================
  Files           106      106              
  Lines          3720     3726       +6     
  Branches        526      526              
============================================
- Hits           1739     1737       -2     
- Misses         1771     1779       +8     
  Partials        210      210

Impacted Files	Coverage Δ	Complexity Δ
...n/java/com/alibaba/csp/sentinel/log/RecordLog.java	50% <0%> (-21.43%)	3 <0> (ø)
...el/slots/block/flow/controller/PaceController.java	78.26% <0%> (-13.05%)	4% <0%> (-1%)
...a/com/alibaba/csp/sentinel/node/StatisticNode.java	66.66% <0%> (+1.75%)	17% <0%> (+1%)	⬆️

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[flystar32]

LGTM