Pure-ldp

* Fixed bugs in update_params() for FreqOracleServer and added update_params() for FreqOracleClient
* Reworked core module structure
* Updated some documentation

* Performance Improvements
* Added Prefix Extending Method (PEM) heavy hitter algorithm that uses OLH
* Added hadamard_response frequency oracle, this is a wrapper of hadamard_response
* Added base classes FreqOracleClient, FreqOracleServer in pure_ldp.core that all Frequency Oracles inherit from
* Improved shared aggregation/estimation logic
* Added more helper methods for frequency oracle servers

Bug fixes:
* Fixed README typos
* Converted exceptions into warnings when using a small privacy budget/estimating on small amounts of data
* Fixed long description in setup.py

pure-LDP is a Python package that provides simple implementations of pure LDP frequency oracles detailed in the paper ["Locally Differentially Private Protocols for Frequency Estimation"](https://www.usenix.org/conference/usenixsecurity17/technical-sessions/presentation/wang-tianhao) by Wang et al

The initial version has implementations of all three main techniques:
1. (Optimal) Unary Encoding - Under pure_ldp.unary_encoding.ue_client and pure_ldp.unary_encoding.ue_server
2. (Summation/Thresholding) Histogram encoding - Under pure_ldp.histogram_encoding.he_client and pure_ldp.histogram_encoding.he_sever
3. (Optimal) Local Hashing - Under pure_ldp.local_hashing.lh_client and pure_ldp.local_hashing.lh_server